It is known that satellite positioning systems comprise a constellation of navigation satellites placed in medium-altitude orbits (of the order of 25 000 km) around the Earth. These navigation satellites and their orbits are generally designated “MEO satellites” and “MEO orbits”, respectively (Medium Earth Orbit) in the art. MEO satellites are distributed uniformly in several orbital planes, so that, at any point on the Earth, a user can see several MEO satellites, that is to say be in direct lines with them (at least three, but four if the user desires to ascertain his altitude) and deduce his own terrestrial coordinates therefrom.
Each MEO satellite carries an atomic clock of high stability and precision, as well as an electronic item of equipment which addresses to the terrestrial users a message comprising the corrected time (relative to a common reference) and the ephemerides of the satellite. On the basis of these messages received from several MEO satellites, the user determines his distance from the various satellites by visibility and therefore deduces therefrom his position in terrestrial coordinates. To ensure nominal operation of the constellation of MEO satellites, a terrestrial control centre performs the measurement of the orbit and of the onboard time for each MEO satellite. This control centre computes the ephemerides of the MEO satellites and the correction of the time to be broadcast by each MEO satellite for the forthcoming times.
The main drawback of these terrestrial-positioning systems is that the quality of the positioning of the terrestrial users is based on the quality of the ephemerides and of the corrected time broadcast by each MEO satellite. So, subsequent to a control centre error or a fault aboard an MEO satellite, it may happen that the real position, the corrected time and/or the ephemerides broadcast by one or more MEO satellites become false, thereby giving rise to a positioning error for all the users in the satellite visibility zone. The error will eventually be detected and corrected by the control centre which monitors the MEO satellites, but, meanwhile, certain users who are using the system for important functions, navigation for example, may be in a situation of serious insecurity.
Several solutions for changing the design of MEO satellites and slaving them to external time references so as to increase the reliability of the messages transmitted by MEO satellites have already been proposed. However, in all these solutions, the detection of anomalies in the MEO satellites is ensured by a network of ground monitoring stations, which transmit to the control centre an information cue regarding these anomalies, and the correction of the latter is entrusted to the said control centre. Additionally, to be effective, these solutions implement at least fifty monitoring stations permanently connected to the said control centre by multiple channels in parallel. This results in high installation and utilization costs.
Moreover, the detection of anomalies by a network of terrestrial monitoring stations gives rise to a strong dilution of precision in retrieving the position of the MEO satellites. Specifically, for a particular MEO satellite, all the distance measurements are performed from terrestrial stations which are therefore all located within a reduced solid angle because the diameter of the Earth is small with respect to the diameter of MEO orbits. The precision in the horizontal plane is consequently limited, correspondingly reducing the reliability in the instantaneous detection of anomalies.
Additionally, all the measurements performed by such a network of terrestrial monitoring stations are disturbed by local effects (tropospheric, ionospheric, multipath errors), thereby causing measurement errors which degrade the reliability of detecting anomalies and risk generating false alerts, if a tolerance threshold very close to these errors has been fixed.
An additional drawback of detecting anomalies by a network of terrestrial monitoring stations stems from the latency time between the moment when an anomaly arises in an MEO satellite and the instant when the user is finally warned of this anomaly. Specifically, the detection/notification chain includes the world network of stations, the control centre, which on the basis of all the measurements of the stations will detect the anomaly, the stations for broadcasting the alert message to the satellites and the system for repeating the alert message aboard the MEO satellite. This long and complex chain induces latency times between the occurrence of the anomaly and the notification to the users that are hardly compatible with the highest operational dependability requirements, such as for example those applicable in the aeronautical field.